ASoC: soc-compress: Initialise delayed work to power down audio

Delayed work was scheduled but not initialised, this patch adds the
actual work and initialises it.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Tested-by: Jeeja KP <jeeja.kp@intel.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>